Naïve B cells engage antigen (Ag) on their B cell receptor (BCR), upregulate CCR7 and migrate to the outer T cell zones to elicit T cell help, leading to differentiation in a T cell dependent or independent manner. T cell independent activation of B cells leads to short-lived IgM antibody secreting plasma cells. However cognate interactions with activated T cells can further augment B cell function and drive follicular differentiation. Congregating T helper cells trafficking through the T cell zones are exposed to the same antigen on MHC class II (MHCII) complexes on antigen presenting cells (APC) such as dendritic cells. Presentation of the antigen-MHCII complex can lead to activation and proliferation of T cells with the relevant antigen receptor. Activated T cells migrate to the B cell areas and engage MHCII-bound Ag on B cells with the same specificity, leading to the formation of an immune synapse. B cells activated in this manner seed the follicles and undergo intense proliferation, differentiating into high-affinity memory B cells or long-lived plasma cells. The criteria for generating high affinity antigen binding B cells through somatic hypermutation of the BCR is controlled by specialist mesenchymal and T cell populations. High affinity antigen binding B cells able to engage and process antigen presented on follicular dendritic cells (FDCs) are selected for further differentiation by interactions with Follicular B Helper T (TFH) cells. Interactions with TFH cells promote the selection of high affinity binders, as well as signalling for differentiation into plasma and long-lived plasma cells. The expression of CXCR5 on B and TFH cells is known to play a key role in optimising these interactions within germinal centres. Specialised microenvironments that foster these T and B cell interactions are created by FDCs that constitutively express CXCL13, which promotes localization and retention of specialized lymphocyte subsets that express CXCR5. In germinal centres, interactions with CXCR5 positive TFH cells are required for the induction of high affinity antibody responses. There is accumulating evidence in the literature that B cells play a role in the pathogenesis of multiple sclerosis (MS) through the production of specific (auto) antibodies that cause myelin destruction. In addition, B cells are involved in all stages of the MS disease process from initiation (antigen capture) to inflammation and tissue damage and could play further roles in MS including, though not limited to, antigen presentation and production of cytokines. In existing MS therapies, B cells have not been directly targeted until recently, with the pilot study using the monoclonal antibody (mAb) rituximab (anti-CD20). However, as many of the current (interferon beta, glucocorticoids, mitoxantrone, natalizumab, fingolimod) and upcoming (alemtuzumab) therapies for MS have the potential to affect B cell behaviour, it is possible that the effects on B cells may contribute to their therapeutic efficacy.
CXCR5 (also known as Burkitt Lymphoma Receptor, i.e. BLR-1, and CD185) is a G-protein coupled seven-transmembrane domain receptors (GPCR 7TM receptor) which is highly expressed on B cells and subpopulations of CD4 T cells. The only known ligand for CXCR5 is the CXC chemokine CXCL13 (also known as BLC or BCA-1). Targeted deletion of CXCR5 indicated that this receptor is involved in B cell migration and localization of B cells in lymph nodes. In the absence of CXCR5, B cells fail to migrate from the T cell rich zones into B cell follicles of spleen with the result that no functional germinal centres are formed. CXCL13 and CXCR5 knockout mice have a similar phenotype, yet interestingly are still capable of mounting a significant antigen specific response albeit much lower than in wild type mice.
CXCL13 is highly expressed in inflamed Central Nervous System (CNS) but is virtually undetectable in normal CNS. Intrathecal production of CXCL13 is thought to be responsible for the recruitment of CXCR5 positive B and T cells into the cerebrospinal fluid (CSF), and the vast majority of B cells in the CSF of MS patients are CXCR5 positive. CXCL13 expression has also been detected in FDCs observed in lymphoid follicle-like structures in the cerebral meninges of patients with secondary progressive MS. The pathogenic B cell response perceived in MS may be the direct product of lymphocyte accumulation in ectopic lymphoid structures which have been shown to modulate B cell function. Furthermore, the close proximity of actively demyelinating lesions may play a major contributory role in driving the chronicity of disease, by generating autoantigen that leads to a persistent autoimmune response. CXCL13 and B-cell activating factor (BAFF; a key regulator of B cell survival) are both markedly and persistently upregulated in the CNS of mice with relapsing remitting or chronic relapsing experimental autoimmune encephalomyelitis (EAE), suggesting that B cell function also plays a role in the chronicity of CNS inflammation in animal models. This finding is consistent with the phenotype of the CXCL13 knockout mice, which exhibit a milder form of EAE compared to wild type controls with rapid resolution of inflammatory symptoms and a complete recovery from disease. These studies have shown that CXCL13 and CXCR5 play an important role in B cell migration, differentiation and proliferative responses. B cells in the vasculature are present in relatively low numbers, therefore positioning within specialised lymphoid microenvironments is critical to interacting with other lymphocyte subsets that dictate B cell effector function. Expression of CXCR5 plays a central role in this process, hence CXCR5 antagonism could potentially block MS disease by reducing recruitment of B and T cells into the CNS, by inhibiting B cell maturation into plasma cells or centrocytes and by blocking interactions with TFH cells and auto-antibody production. Perhaps significantly, CXCR5 blockade could disrupt ectopic lymphoid follicle development by attenuating auto-antigen presentation within these sterile lymphoid environments, and promote resolution of the inflammatory process.
The generation of ectopic lymphoid structures, through expression of CXCL13 and interactions with CXCR5 bearing cells has also been identified in other chronic inflammatory diseases such as rheumatoid arthritis (RA) and Sjögren's syndrome and more recently CXCR5 expression has also been shown in a number of pancreatic carcinomas. Thus, antagonism of CXCR5, via the use of antibodies directed to CXCR5, may also be a useful therapeutic approach in these diseases. This is credible in view of the similarity of expression between CXCL13 (the ligand of CXCR5) and BAFF in the CNS of mice with EAE as disclosed above, and because anti-BAFF therapy is well-established for the treatment of inflammatory/autoimmune disorders. Indeed, different anti-BAFF antibodies are on the market or are being developed for the treatment of inflammatory/autoimmune disorders: for instance, belimumab, already approved for systemic lupus erythematosus (SLE), has also been assessed for diseases such as MS, RA and Sjogren's syndrome, and tabalumab is currently in clinical trials for SLE and MS.
As described above, the CXCL13/CXCR5 pathway plays a key role in B cell functions. Burkle et al. (2007) have also shown the involvement of this pathway in cancer, such as in B-cell chronic lymphocytic leukaemia (B-CLL). It was notably shown that CLL patients have significantly higher serum levels of CXCL13 than healthy patients. The authors were also able to show that anti-CXCR5 antibodies inhibit chemotaxis to CXCL13, suggesting that CXCR5 is a novel therapeutic target for patients with CLL.
WO2009032661 describes humanized antibody polypeptides that specifically bind to the extracellular domain of human CXCR5. It also describes methods of treating a patient having a disorder involving CXCR5 positive cells, comprising administering to said patient a CXCR5 antagonist, which binds CXCR5.
Considering the major impact of inflammatory and/or autoimmune diseases, as well as cancers on public health, there is thus a need for novel molecules that could be useful as medicaments notably for treating inflammatory and/or autoimmune diseases, such as multiple sclerosis, rheumatoid arthritis or Sjögren's syndrome, as well as cancers such as pancreatic carcinomas, B-CLL or cancer/lymphomas involving the CXCR5/CXCL13 pathway.